Conventionally, a computer system providing a large-scale data storage service to the host devices exists. This system is known to include host devices, a storage apparatus to which the host devices are connected, and a management device of the storage apparatus.
The storage apparatus manages multiple hard disks by the RAID (Redundant Array of Independent/Inexpensive Disks) method. The storage apparatus logicalizes physical storage areas included in multiple hard disks and provides them as logical volumes to the host devices. The host devices access the logical volumes and request reading and writing of data.
One of the logicalization technologies of this type is called thin provisioning. The storage apparatus sets a virtual volume, which does not include any physical storage areas, for the host devices. As the host devices make write accesses to the virtual volume, the storage apparatus sequentially allocates storage areas to the virtual volume.
Therefore, this technology is more effective, compared with the method of allocating a large capacity storage area to logical volumes from the beginning, in that storage resources can be efficiently utilized. This technology is disclosed in Japanese Patent Application No. 2003-015915A and Japanese Unexamined Patent Application Publication No. 2006-338341A.
The means for providing storage areas to the virtual volume consists of a logical group, called a pool, of multiple logical volumes to each of which physical storage areas are allocated. Each of the multiple logical volumes is called a pool volume. If there is a write access from the host device to the virtual volume, the storage area in the corresponding area of the pool volume is allocated to the access destination of the virtual volume.
The storage apparatus can store write data from the host device by making the area accessed by the host device for the virtual volume correspond with the area in the pool volume to which the storage capacity is allocated.
Though the storage apparatus intends to allocate storage areas to the multiple pool volumes equally, with the passage of time, the problem arises that this equality is impaired. Therefore, Japanese Patent Application No. 2008-234158A discloses that, by migrating storage areas among multiple pool volumes, the storage area capacity becomes equal among the multiple pool volumes.